Loss of signal in an optical network may result, for example, from a cut to an optical fiber or a failure of terminal equipment. When a loss of signal occurs in a branched optical network, it can be very difficult to distinguish between these two types of failure. Knowing the cause of a loss of signal is important because it permits a service provider to determine, for example, whether to dispatch a repair crew and what type of training and equipment that may be needed by that repair crew.
An example of such a branched optical network is shown in FIG. 1. Shown in that figure is an example of what is often called a xe2x80x9cpassive optical networkxe2x80x9d (PON) 100. In the PON shown in FIG. 1, a single fiber 110 connects a central office 120 to a passive optical splitter 130 that may be located in a remote node. From an output of the splitter 130, individual optical fibers 135(1) . . . 135(n) are further connected to a respective individual optical networking unit (ONU) 140. As is known, an ONU may serve a single home, several homes, or an entire building or residences or offices (not shown).
In this example, xe2x80x9cdownstreamxe2x80x9d transmission (from central office 120 to optical networking unit 140) occurs at 1.5 xcexcm, and xe2x80x9cupstreamxe2x80x9d transmission occurs at 1.3 xcexcm from transmitters 121 and 142 respectively. Corresponding upstream and downstream reception occurs at corresponding receivers 141 and 122. Upstream and downstream signals are separated using 1.5/1.3 xcexcm coarse wavelength division multiplexers 143 and 123, respectively.
One prior art method of determining whether an optical fiber break has occurred is to use optical time domain reflectometry (OTDR). As can be appreciated by those skilled in the art, a broken optical fiber will cause a reflection that may be detected through the use of OTDR. Unfortunately, OTDR is not fool-proof in optical networks. For example, in the PON shown in FIG. 1, if there is a loss of signal from an individual ONU connected on one of the individual optical fibers 135(1) . . . 135(n), an OTDR signal sent from the central office 120 would superimpose reflected signals resulting from breaks in multiple branches of the network into a single reflected signal, thereby making the reflected signal ambiguous as to which particular branch contains a broken optical fiber.
In an article entitled xe2x80x9cFauIt Location Technique for In-Service Branched Optical Fiber Networksxe2x80x9d, that appeared in IEEE Photon. Tech. Left., vol. 2, pp. 766-768, 1990, I. Sankawa, S. I. Furukawa Y. Koyamada and J. Izumita suggested that one can overcome this ambiguity by collecting OTDR traces prior to a failure and storing the collected traces in computer memory for future reference. When a failure occurs, an OTOR trace may be compared with a stored trace in an attempt to determine whether or not a fiber break has occurred. Drawbacks to such an approach are numerous. Specifically, the approach 1) requires the storage of a number of OTDR traces at a central or otherwise accessible location that may be serving a large number of networks; 2) the traces will have to be updated frequently enough to ensure their accuracy; and 3) sophisticated operators or computer algorithms are needed to correctly interpret the OTDR traces.
An alternative approach is to send a repair crew to the ONU whenever a failure is detected. Of course, such a repair crew must be properly trained and equipped both for ONU replacement and in using OTDR. While such a repair crew may in fact correctly isolate and repair a failure in the optical network, it is nevertheless desirable to understand the nature of the failure prior to dispatching the repair crew.
Consequently a continuing need exists for methods that facilitate fault identification in optical networks and in particular, branched optical networks.
The above problems are overcome and advance is made over the prior art in accordance with the principles of my invention directed to a method for identifying faults in a branched optical network. The method involves the transmission of an optical signal from a central office to a plurality of optical network units along a plurality of optical paths within a branched optical network. Selectively, portions of the optical signal are reflected back to the central office from modulators situated within the optical network units. From these reflected signals, the method advantageously determines the existence of faults within the branched optical network.
In accordance with the present invention, the modulators may be micro-mechanical, anti-reflective modulator switches (MEMS) devices, thereby permitting a variety of selection and determination methods.
Further features and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention are described in detail below with reference to the accompanying drawing.